A gradient-type furnace for growing semiconductor crystals is illustrated in U.S. Pat. No. 4,086,424 which issued Apr. 25, 1978. In this patented furnace a temperature profile having 21 zones is achieved by sequentially powering the heating elements in each zone. An improved furnace is disclosed in U.S. patent application Ser. No. 132,224, filed Dec. 14, 1987, and assigned to the present assigned. Other gradient-type furnaces of the type employed with the present invention are disclosed in U.S. patent applications No. 264,341, filed Oct. 31, 1988; and No. 264,378, filed Oct. 31, 1988.
The process for fabricating a semiconductor crystal, such as gallium arsenide, starts with an ampoule which contains a raw gallium arsenide charge which is not of single crystal composition. A seed of single crystalline structure is positioned within the ampoule and adjacent the ingot. The ampoule containing the charge is then subjected to a sequence of carefully controlled temperature changes which result in the melting and solidification of the charge. The presence of the seed adjacent the charge causes the solidification of the charge as a single crystal structure.
In order to achieve high uniformity and homogeneity of the grown crystal, it is desirable to have the crystal grown in space where it is not subjected to normal gravitational forces. However, due to the excessive vibration during lift-off and re-entry, the seed and charge are often disturbed upon lift-off so that crystal growth is not possible in space or the grown crystal is damaged when encountering vibrational and impact forces upon re-entry and landing.